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If f
5
=
B/4 (i.e. 2 f
5
=
B/2), the component f
0
-2 f
5
is attenuated by 3 dB,
which introduces a meter error of 1.65 dB.
If f
5
=
B/2 (i.e. 2 f
5
=
B), the component f
0
-2 f
5
attenuated by 18 dB,
which introduces a meter error of 0.1 dB.
Finally if f
5
=
B, the attenuation of the f
0
-2 f
5
component is greater
than 40 dB, and the error in the meter reading is negligible.
9.4. EFFECT OF RIPPLE IN V.C.O. CONTROL VOLTAGE ON ANAL VSIS
RECORDING AND B.F.O. OUTPUT
Ripple in the external frequency control voltage can produce unwanted
modulation of the variable frequency fv Issued by the V.C.O.
The frequency, fv, is mixed in the first modulator with the incoming
signal for analysis (V
5
sin (2rr f
5
t))
.
Sum and difference frequencies are
produced.
The
difference
frequency
is
removed
by
filteriny,
see
Section 5.3.4. Therefore, only the sum frequency is important, where
If ripple, Vm, of frequency fm, is present in the control voltage, side
bands are produced in the sum frequency.
vsum
vs sin (2
1T
(fs +f) t)
+
~
mV
5
sin(2rr(f
5
+(fv+fm))t)
+
+
m V
5
sin (2
1T
(f
5
+
(fv
-fm)) t),
where m is the modulation index
and
m
fm
The amplitude of the side bands are
~
m · V s
89
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